Chapter 5
Electrical Circuit Design for Commercial and Industrial Occupancies

ARTICLE 220^*

*References in the margins are to the specific applicable rules and tables in the National Electrical Code.

The design of the electrical circuits for commercial and industrial occupancies is based on specific Code rules that relate to the loads present in such occupancies. The design approach is to separate the loads into those for lighting, receptacles, motors, appliances, and other special loads and apply the applicable Code rules. In general, the loads are considered continuous unless specific information is given to the contrary.

215.5

If the electric load is large, the main service may be a three-phase supply which supplies transformers on the premises. It is not uncommon to have secondary feeders supplying panelboards which, in turn, supply branch circuits operating at different voltages. In this case, the design of the feeder and branch circuits for each voltage is considered separately. The rating of the main service is based on the total load with the load values transformed according to the various circuit voltages if necessary. A feeder circuit diagram is essential when loads of different voltages are present.

TABLE 220.42,
TABLE 220.56
220.86,
220.88
220.102

In most commercial and industrial occupancies, no demand factors are applied to the loads served.¹ The lighting load in hospitals, hotels and motels, and warehouses, however, is subject to the application of demand factors. In cafeterias and similar establishments the load of electric cooking equipment is subject to a demand factor if there are more than three units. Optional calculation methods to determine feeder or service loads for schools, restaurants, and farms are also provided in the Code.

¹Demand factors are allowed for feeders supplying receptacle circuits in nondwelling occupancies. These demand factors may be applied to receptacles for general use (i.e., not continuous loads) and where allowed by local authorities.

220.44

ARTICLE 550
ARTICLE 551
ARTICLE 555

Special occupancies, such as mobile homes and recreational vehicles, require the feeder or service load to be calculated in accordance with specific Code rules. The service for mobile home parks and recreational vehicle parks is also designed based on specific Code rules that apply only to those locations. The feeder or service load for receptacles supplying shore power for boats in marinas and boatyards is also specified in the Code.

Most situations for which specific Code rules are provided are covered in this chapter either by example or as a quiz problem. Other Code rules that pertain to wiring system design are summarized in Table 5–1 at the end of this chapter.

Table 5–1. Miscellaneous Wiring System Design Rules

90.1
210.19(A),
215.2

Practical Electrical Design. The design examples given in the Guide represent a design based on Code requirements. In many cases, the equipment ratings that result from strict application of Code rules are not adequate for a specific practical situation, and this is recognized by the Code. The conductor sizes that result from calculations performed in accordance with Code rules do not take into account voltage drop caused by the length of conductors. The Code recommends that the total voltage drop from the service to the farthest branch-circuit outlet should not be greater than 5%. This problem and considerations for future expansion are recognized by the Code.

The circuit voltages are taken to be nominal voltages based on the main service voltage or a stepped-down voltage at a transformer. These voltages may be higher than the actual voltage at an outlet (which may be lower as a result of voltage drop on the conductors). For example, a 480-volt service may actually supply 460 volts or even 440 volts to the farthest outlet. A conservative design approach would be to use the lower voltage in the calculations and thereby obtain a larger required ampacity for the conductors. The voltages to be used in the Guide are, of course, those given in the problem under discussion.

240.6

Sizes and Ratings. Transformers used in lighting and power circuits are normally considered capable of carrying their full-rated load continuously. Thus, a 10-kilowatt continuous load requires service conductors rated for 1.25 × 10 kilowatts = 12.5 kilowatts, but a transformer rated at only 10 kilovolt-amperes. In practice, transformers are manufactured with a limited number of standard ratings.

240.6

Fuses used to protect circuits are supplied in the standard sizes listed in the Code. If the current exceeds the fuse size for a nontime-delay fuse, the fuse element will open. Many circuit breakers, especially those for low current values, have a fixed rating or setting. Other breakers for larger loads (100 to 6000 amperes) are supplied in standard size frames such as 100, 225, 400, 600, 800, 1200 amperes, etc. The tripping mechanism that determines the rating or setting can be adjusted at the factory and the standard ratings given in the Code can be selected (although all ratings are not readily available commercially). The Code rules that apply to the rating or setting of overcurrent devices refer to the capacity of the tripping mechanism in the breaker if adjustable breakers are used.

Service-entrance panelboards protected by main circuit breakers are also produced with standard capacities that correspond to the standard circuit breaker frame sizes. The circuit breaker setting is adjusted to protect the circuit as determined by Code rules.

5–1 Typical Commercial Occupancy Calculations

Typical commercial establishments range from small stores with single-phase services to large office buildings with three-phase services and significant motor loads. The design of the electrical wiring system for such occupancies is treated in this section. The examples given become increasingly complex although the approach to the design problem is the same for all examples. The branch circuits are designed first, then the feeders if used, and finally the main service.

450.3

When transformers are not involved, a relatively simple design problem with a single voltage results, as in the first example in this section. If step-down transformers are used, the transformer itself must be protected by an overcurrent device which may also protect the circuit conductors in most cases. In any design, the rules for the protection of the transformer must be considered to assure a design in complete conformance with Code rules.

ARTICLE 408

Switchboards and panelboards used for the distribution of electricity within a building are also subject to Code rules. In particular, a lighting and appliance panelboard cannot have more than 42 overcurrent devices to protect the branch circuits originating at the panelboard. This rule could affect the number of feeders required when a large number of lighting or appliance circuits are needed.

5–1.1 Design Example for a Store with Show Window

A small store has outside dimensions of 50 feet by 60 feet and a 24-foot length of show window. The store has 20 general-purpose receptacles that do not supply continuous loads. The branch circuits and the 120/240-volt service for the building are to be designed using THW copper conductors. Twenty-ampere circuits are used for all branch circuits.

220.12
ANNEX D
EXAMPLE 3
600.5

The design summary is shown in Figure 5–1. The branch circuits for lighting and receptacles required a total of seven 20-ampere circuits. The load for the 24-foot show window is computed on the basis of 200 volt-amperes per linear foot and it is considered a continuous load. The 6000-volt-ampere load requires three 20-ampere circuits. One additional 20-ampere circuit is needed for the required outside outlet for sign or outline lighting if the store is on the ground floor.

Figure 5–1. Design Summary for Store with Show Window

220.40 210.19(A) TABLE 310.16 230.90 240.6 250.66

The feeder or service load in this example is simply the sum of the branch-circuit loads. If it is assumed that the sign circuit is to be continuously loaded, its maximum load could be as high as .8 × 120 volts × 20 amperes = 1920 volt-amperes. Since the actual load is not given, a 1200 volt-ampere minimum load may be used in the calculation. The total service rating is then 22 050 volt-amperes, or 91.9 amperes at 240 volts. The standard 100-ampere overcurrent protective device would be used which requires No. 3 conductors. For comparison the connected load is shown to be 18,600 VA ÷ 240 V = 77.5 amperes. The service-entrance conductors would be enclosed in a 1-inch conduit and a No. 8 grounding electrode conductor would be used at the service.

5–1.2 Design Example for an Office Building

A 14,000-square-foot office building is served by a 460Y/265-volt, three-phase service. The building contains the following loads:

a. 10,000-volt-ampere, 208/120-volt, three-phase sign

b. 80 duplex receptacles supplying continuous loads²

220.44

²If the receptacle load is not considered continuous, the portion of the receptacle load over 10 kVA may be subjected to a demand of 50%.

c. 24-foot-long show window

d. 12-kilowatt, 208/120-volt, three-phase electric range

e. 10-kilowatt, 208/120-volt, three-phase electric oven

f. 20-kilowatt, 460-volt, three-phase hot-water heater

g. Fifty 150-watt, 120-volt lighting fixtures for outside lighting

h. One hundred fifty 200-volt-ampere, 265-volt fluorescent fixtures for interior office lighting

i. 7.5-horsepower, 460-volt, three-phase air handler motor protected by an approved instantaneous-trip circuit breaker

j. 35-kilowatt, 460-volt, three-phase electric heating unit

k. 50-ampere, 460-volt, three-phase air-conditioning unit

The ratings of the service equipment, transformer, and the feeders or branch circuits supplying the equipment are to be determined. Circuit breakers are used to protect each circuit and THW copper conductors are used throughout the wiring system. The continuous load currents are used to calculate the ampacity of the service conductors.

240.6 TABLE 310.16

A simplified feeder diagram is shown in Figure 5–2. The 208Y/120-volt lighting panel is designed first as shown in Figure 5–3(a). The continuous load on the panel is 73,375 volt-amperes, or 204 amperes at 208Y/120 volts. The 208Y/120-volt lighting panel may be protected by a standard-size 225-ampere circuit breaker (L_B). The conductors are No. 4/0 type THW copper.

Figure 5–2. Simplified Feeder Diagram for 14,000-Square-Foot Office Building

Figure 5–3(a). Calculations for 208Y/120-Volt Loads

450.3(B)

Since the transformer supplying the lighting panel is rated for a continuous load, the kilovolt-ampere rating need only be 58.7 kilovolt-amperes. A commercially available 75-kilovolt-ampere transformer would be selected. The transformer requires individual overcurrent protection set at not more than 125% of the rated primary current. The rated primary current for the 75-kilovolt-ampere transformer is

250.20

The maximum setting of the transformer overcurrent protective device (T_B) is then 1.25 × 94.1 amperes = 117.6 amperes for which a standard 125 A device could be used. However, the primary conductors must be protected at their ampacity if their ampacity is lower.

250.30 TABLE 250.66

The 208Y/120-volt circuit is a separately derived system from the transformer and is grounded by means of a grounding electrode conductor which must be at least a No. 2 copper conductor based on the No. 4/0 copper feeder conductors.

Design calculations for the primary feeder and the other 460/265-volt circuits are summarized in Figure 5–3(b).

Figure 5–3(b). Feeder or Branch Circuits for 460/265-Volt Loads

The 460-volt feeder that supplies the transformer carries a continuous load of

which requires a 100-ampere protective device and three No. 3 THW copper conductors.

TABLE 310.16 240.6

The 265-volt lighting feeder load is the larger of the area load of 3.5 volt-amperes per square foot of building area or the actual load of the 150 fluorescent lighting fixtures. The area load is 1.25 × 3.5 volt-amperes/square foot × 14,000 square feet = 61,250 volt-amperes or a balanced load of 76.9 amperes. The actual load is 1.25 × 200 volt-amperes × 150 = 37,500 volt-amperes and is neglected. The feeder of No. 4 THW copper conductors is protected by a 90-ampere circuit breaker.

TABLE 310.16 430.52

The continuous hot-water heater requires a circuit rating of at least 31.4 amperes; therefore, a 40-ampere protective device and No. 8 conductors are used for the circuit. The circuit supplying the electric heating requires a circuit ampacity of at least 54.9 amperes; therefore, a 60-ampere protective device and No. 6 conductors are selected.

TABLE 430.250, TABLE 430.52 440.32

An instantaneous-trip circuit breaker was selected to protect the 7.5-horsepower motor circuit. This arrangement is allowed if the breaker is adjustable and is part of an approved controller. A full-load current of 11 amperes requires No. 14 conductors protected by a breaker set as high as 8 × 11 = 88 amperes. A standard 80-ampere setting could be selected.

440.22 220.40

The air-conditioning circuit must have an ampacity of 1.25 × 50 amperes = 62.5 amperes for which No. 6 conductors are selected. An overcurrent protective device for this circuit cannot be set higher than 1.75 × 50 amperes = 87.5 amperes.

430.63

The design calculations for the main service are summarized in Figure 5–3(c). The loads are balanced and could be computed in terms of volt-amperes, but the loads are computed here in terms of amperes as is typically done with commercial and industrial systems in which unbalanced loads are usually present. Calculation of the load in amperes also simplifies the selection of the main overcurrent protective device which is based on the setting of the largest motor branch-circuit protective device plus the sum of the other loads in amperes.

Figure 5–3(c). Main Service Load Calculation

250.24

The 73,375-volt-ampere, 208/120-volt lighting panel load represents a line load of 92.1 amperes at 460 volts. Even though no neutral load is present because the primary of the transformer is a 460-volt, three-phase delta winding, a neutral capacity at least equal to that of the grounding electrode conductor must be provided to the service.

TABLE 310.16

The 265-volt interior lighting load is 61,250 volt-amperes. As a balanced load, the line load is 76.9 amperes and a neutral with this ampacity is required. The three-phase loads contribute a load in amperes according to their rating as shown in Figure 5–3(c). Twenty-five percent of the largest motor load is added, resulting in a total service load of 273.9 amperes.

250.24 TABLE 250.66

The ungrounded conductors are selected to be 300-kcmil type THW copper conductors. The neutral ampacity must be at least 76.9 amperes according to the calculation, but the neutral conductor cannot be smaller than the grounding electrode conductor. The neutral and the grounding electrode conductor at the service are therefore selected to be No. 2 copper based on the 300-kcmil service-entrance conductors.

430.63 CHAPTER 9

The rating or setting of the overcurrent protective device is based on the rating of the largest motor branch-circuit protective device (90 amperes) plus the sum of the currents of the other loads. The maximum setting permitted is 301.4 amperes. A standard 300-ampere device is selected as the next lower standard size. The conduit must enclose three 300-kcmil type THW conductors and one No. 2 THW conductor; therefore, a 2½-inch conduit is required.

5–1.3 Design Example for an Office Building with Warehouse

An office building and a 40,000-square-foot warehouse are supplied by a 240/120-volt, three-phase, four-wire service. The service-entrance equipment is located in the office building. The warehouse has its own feeder panel with overcurrent protection and a disconnecting means. The feeder panel is within 25 feet of the service-entrance equipment. Instantaneous-trip breakers protect the motor circuits.

The office building contains the following loads:

a. 10,000 square feet of area lighted by 120-volt incandescent lamps

b. 240 duplex receptacles for noncontinuous loads

c. Six -horsepower, 240-volt, single-phase motors on separate circuits

d. 28-ampere, 240-volt, three-phase air-conditioner motor-compressor

e. 1-horsepower, 240-volt, three-phase air handler motor

f. Outside sign circuit connected between phase A and neutral

The service and feeder overcurrent protective devices are fuses and the other circuits are protected by circuit breakers. The phase A and C conductors are parallel 350-kcmil type THW copper conductors in two conduits to provide for future expansion.³

220.87 430.62(B)

³The Code provides for future expansion and also allows an optional calculation to compute the amount of additional load that may be added to an existing electrical installation.

The loads supplied from the warehouse panel are:

a. 40,000 square feet of area lighted by 120-volt incandescent lamps in heavy-duty type lampholders

b. 40 duplex receptacles (considered continuous)

c. Three 1½-horsepower, 240-volt, three-phase motors

d. 30-ampere, 240-volt, single-phase special appliance

240.21(B)

The feeder for the warehouse is tapped from the main service-entrance conductors and is enclosed in conduit. As long as the distance is within 25 feet and the feeder conductors have an ampacity of not less than one-third that of the service overcurrent protective device, a set of fuses in the warehouse feeder panel may protect the feeder conductors. This so-called 25-foot tap rule eliminates the need for feeder overcurrent protective devices within the service equipment.

A simplified distribution system diagram is shown in Figure 5–4. The conductor ampacities of the service-entrance conductors and the feeder or branch circuits are to be determined as well as the ratings of the required overcurrent protective devices.

Figure 5–4. Distribution System of Office Building and Warehouse

Warehouse Loads. The design of the warehouse branch circuits is summarized in Figure 5–5(a). Each of the three 1½-horsepower motor circuits is protected by a 40-ampere overcurrent protective device. All of the loads are considered to be continuous loads.

Figure 5–5(a). Calculations for Warehouse Branch Circuits and Feeder

220.40 240.21 430.63 240.6

The calculations for the warehouse protective devices are also shown in Figure 5–5(a). Although the total ampacity is calculated, the feeder conductor sizes cannot be determined until the service overcurrent devices are selected because of the requirements of the 25-foot tap rule. The total load on phases A and C is 146.5 amperes and only 19.5 amperes on phase B. The overcurrent protective devices must be rated or set at not more than 189 amperes for phases A and C and 62 amperes for phase B based on the largest branch-circuit protective device (50 amperes) plus the sum of the other loads. Two 175-ampere fuses and one 60-ampere fuse could be used.

Office Loads. Figure 5–5(b) presents the design calculations for the other feeders and branch circuits originating at the service equipment and the branch circuits originating from the 240/120-volt lighting and appliance panelboards supplied from the service equipment.

Figure 5–5(b). Calculations for Branch Circuits and Feeders Originating from Main Service Equipment

600.5 430.53

The branch circuits for the 120-volt lighting load and receptacle load are determined as shown in part A of Figure 5–5(b). Phases A and C and the neutral supply these loads. A minimum load of 1200 volt-amperes is assumed for the 20-ampere sign circuit which is supplied only by phase A and the neutral. The six -horsepower motors are supplied by six individual circuits in the example although several motors could be supplied by a single 15-ampere, 240-volt circuit.

The individual branch circuits for the three-phase motor loads are also shown in part B of Figure 5–5(b). The 1-horsepower motor and the 28-ampere motor-compressor are supplied by 40-ampere and 45-ampere circuits, respectively.

408.35 220.40 240.4

Two or more 240/120-volt lighting and appliance circuit panelboards are required since each panelboard may house 42 overcurrent devices with each pole of a circuit breaker counting as one device. The feeder or feeders supplying the panelboards must have a total overcurrent protection of 316.9 amperes for phase A as shown in part C of Figure 5–5(b). The conductor size would be based on the loads and the requirements for conductor overcurrent protection.

300.20 310.4 TABLE 310.16

The main service load consists of the 240/120-volt loads, the three-phase motor loads, and the warehouse load as shown in Figure 5–5(c). The total loads are 500.5 amperes, 57.2 amperes, and 490.5 amperes for phases A, B, and C, respectively. The loads on phases A and C are carried by two 350-kcmil type THW copper conductors in parallel per phase. If two conduits are used to enclose the conductors, then phase B and the neutral conductor must also be included in each conduit. Although the required ampacity of the phase B conductor is only 57.2 amperes, two No. 1/0 conductors must be used because that is the smallest size conductor permitted for use as parallel conductors. The neutral of each circuit must have an ampacity of at least ½ × 334.8 amperes = 167 amperes; thus, two No. 2/0 THW copper conductors in parallel would be used for the neutral. Two 2½-inch conduits would be used to enclose the service-entrance conductors.

Figure 5–5(c). Service Load Calculation for Office Building and Warehouse

430.63 430.62(B)

The main fuses are selected based on the 45-ampere circuit breaker for the motor-compressor plus the sum of the other loads. The allowed fuse sizes are 600 amperes for phase A, 600 amperes for phase C, and 60 amperes for phase B. Since this is a motor feeder, strict adherence to Code rules requires the next smaller standard-size overcurrent protective device to be used. The Code, however, allows the rating or setting of the overcurrent protective device to be based on the rated ampacity of the conductors whenever higher capacity feeders are installed in order to provide for future additions or changes. In this case, since the 350-kcmil conductors for phases A and C can carry 2 × 310 amperes = 620 amperes, 600-ampere fuses could be used for both phases and thereby provide capacity for future additions.

240.21(B)

Feeder Tap to Warehouse. The ampacity of the tap conductors must be at least one-third of the overcurrent protection for the paralleled service-entrance conductors. The minimum tap conductor ampacity then would be 200 amperes for phases A and C. Phase B requires a conductor with a 60-ampere ampacity based on the 60-ampere fuse in the warehouse panel. The neutral load is 89.5 amperes. Two No. 3/0, one No. 6, and one No. 3 conductors would be required.

250.32 TABLE 250.66

A grounding electrode conductor is required at the warehouse and must be at least a No. 2/0 copper conductor based on the equivalent area of the two 350-kcmil conductors.

QUIZ 5–1 (Closed-Book)

1. A main feeder uses No. 6 type THW copper conductors (65-ampere ampacity). A suitable tap circuit less than 25 feet long must have an ampacity of at least: (a) 65 amperes (b) 22 amperes (c) 32.5 amperes 2. A 240-volt, two-wire service is required for a building. The main utility circuit is a 120/240-volt circuit. Is a neutral conductor required for the 240-volt service? If so, how is it sized? 3. A secondary feeder is supplied by a 10-kilovolt-ampere, 480/240-volt, single-phase transformer. What is the largest primary feeder overcurrent device that may be used so that individual primary overcurrent protection for the transformer is not required? What is the maximum rating of the secondary side overcurrent protective device on the secondary side?

(Open-Book)

1. Design the circuits for an office building having three floors and a basement. The main service is a 460Y/265-volt circuit. Each floor contains the following: (a) 22,000 square feet of area (b) 265-volt fluorescent ceiling lighting supplied by a 460/265-volt panel with 30-ampere circuits (c) 120-volt duplex receptacles supplied by a 208Y/120-volt panel with 20-ampere circuits. Load is 1 VA per square foot. (d) A 208Y/120-volt transformer to supply the receptacles The basement has a 5000-square-foot storage area containing the following equipment: (a) A 10,000-watt, 120-volt lighting and receptacle load for the machinery room on 20-ampere circuits (load is continuous) (b) Three 50-horsepower, 480-volt, three-phase synchronous motors each protected by time-delay fuses (c) Two 15-horsepower, 480-volt, three-phase squirrel-cage motors (d) Five 10-horsepower, 480-volt, three-phase squirrel-cage motors Draw a feeder diagram and select the circuits, overcurrent protective devices, and transformer ratings for the building.

5–2 Feeder and Service Design For Other Commercial Occupancies

This section discusses the design of feeder or service equipment for establishments with commercial cooking facilities, motels and hotels, schools, and farms. The design problem for each of these occupancies differs from previous examples because certain Code rules must be followed that do not apply generally.

5–2.1 Optional Calculation for New Restaurants

220.88 TABLE 220.88

An optional method is provided in the Code to calculate the service load of a new restaurant. To apply the method, the total load in kilovolt-amperes is calculated by adding the contributions from each of the connected loads. If the restaurant contains all electric equipment, a demand factor of 80% is applied to the total load up to 200 kilovolt-amperes. Other demand factors are applied to the all electric load if it is over 200 kilovolt-amperes. The demand factor for a restaurant with nonelectric cooking equipment is 100% if the load does not exceed 200 kilovolt-amperes. Other demand factors apply to a total load greater than 200 kilovolt-amperes as indicated in Figure 5–6.

Figure 5–6. Comparison of Optional and Standard Service Calculation for a Restaurant

Design Example. A restaurant with nonelectric ovens has the following loads:

a. 99 kilovolt-amperes of lighting and other loads considered continuous

b. 62 kilovolt-amperes of miscellaneous loads

c. 341 kilovolt-amperes of air-conditioning equipment

d. 25 kilovolt-amperes of refrigeration equipment

e. 34 kilovolt-amperes of food preparation and cleaning equipment consisting of more than six units

220.10, 220.20 220.88

Figure 5–6 compares the calculated service load, using both the standard and optional methods. The total demand using the standard method results in a 1600-ampere service. Applying the optional method, the total connected load of 561 kilovolt-amperes is reduced to 368.7 kilovolt-amperes using the demand calculation. The service current in this case is 1023 amperes. The neutral conductor load can be computed using the standard method.

5–2.2 Services for Hotels and Motels

TABLE 220.42 TABLE 220.14(J)

The portion of the feeder or service load contributed by general lighting in hotels and motels without provisions for cooking by tenants is subject to the application of demand factors. In addition, the receptacle load in the guest rooms is included in the general lighting load at 2 watts per square foot. The demand factors, however, do not apply to any area where the entire lighting is likely to be used at one time, such as the dining room or a ballroom. All other loads for hotels or motels are calculated as shown previously.

TABLE 220.12

Simplified Design Example. It is required to determine the 120/240-volt feeder load contributed by general lighting in a 100-unit motel. Each guest room is 240 square feet in area. The general lighting load is

2 VA/ft² × 240 ft²/unit × 100 units = 48,000 VA

TABLE 220.42

but the reduced lighting load is

This would be added to any other loads on the feeder or service to compute the total capacity required.

5–2.3 Optional Calculation for Schools

220.86

The Code provides an optional method for determining the feeder or service load of a school equipped with electric space heating or air conditioning, or both. This optional method applies to the building load, not to feeders within the building.

The optional method for schools basically involves determining the total connected load in volt-amperes, converting the load to volt-amperes/square foot, and applying the demand factors from the Code table. If both air-conditioning and electric space-heating loads are present, only the larger of the loads is to be included in the calculation.

Simplified Design Example. A school building has 200,000 square feet of floor area. The electrical loads are as follows:

a. Interior lighting at 3 volt-amperes per square foot

b. 300-kilovolt-ampere power load

c. 100-kilovolt-ampere water heating load

d. 100-kilovolt-ampere cooking load

e. 100-kilovolt-ampere miscellaneous loads

f. 200-kilovolt-ampere air-conditioning load

g. 300-kilovolt-ampere heating load

The service load in volt-amperes is to be determined by the optional calculation method for schools.

As shown in Figure 5–7, the combined connected load is 1500 kilovolt-amperes. Based on the 200,000 square feet of floor area, the load per square foot is

Figure 5–7. Optional Calculation for school

TABLE 220.86

The demand factor for the portion of the load up to an including 3 volt-amperes/square foot is 100%. The remaining 4.5 volt-amperes/square foot in the example is added at a 75% demand factor for a total load of 1,275,000 volt-amperes.

TABLE 220.55 NOTE 5 220.56

Household Electric Cooking Appliances in Instructional Programs. If the standard method is used to compute the feeder or service load for a school, the Code range table may be used to compute the load for household cooking appliances used for instructional programs. The kitchen equipment load in the school cafeteria may also be reduced if there are three or more units.

5–2.4 Feeder and Service Calculations for Farms

ARTICLE 220, PART V

The Code provides a separate method for computing farm loads other than the dwelling. Tables of demand factors are provided for use in computing the feeder loads of individual buildings as well as the service load of the entire farm.

220.102

The demand factors may be applied to the 120/240-volt feeders for any building or load (other than the dwelling) that is supplied by two or more branch circuits. All loads that operate without diversity, that is, the entire load is on at one time, must be included in the calculation at 100% of connected load. All “other” loads may be included at reduced demands. The load to be included at 100% demand, however, cannot be less than 125% of the largest motor and not less than the first 60 amperes of the total load. In other words, if the nondiverse and largest motor load is less than 60 amperes, a portion of the “other” loads will have to be included at 100% in order to reach the 60-ampere minimum.

220.103

After the loads from individual buildings are computed, it may be possible to reduce the total farm load further by applying additional demand factors.

Design Example. A farm has a dwelling and two other buildings supplied by the same 120/240-volt service. The electrical loads are as follows:

a. Dwelling—100-ampere load as computed by the calculation method for dwellings

b. Building No. 1—

1. 5-kilovolt-ampere continuous lighting load operated by a single switch

2. 10-horsepower, 240-volt motor

3. 21 kilovolt-amperes of other loads

c. Building No. 2—

1. 2-kilovolt-ampere continuous load operated by a single switch

2. 15 kilovolt-amperes of other loads

Determine the individual building loads and the total farm load as illustrated in Figure 5–8. The nondiverse load for building No. 1 consists of the 5-kilovolt-ampere lighting load and the 10-horsepower motor for a total of 83.3 amperes. This value is included in the calculation at the 100% demand factor. Since the requirement for adding at least the first 60 amperes of load at the 100% demand factor has been satisfied, the next 60 amperes of the 87.5 amperes from all other loads are added at a 50% demand factor and the remainder of 87.5 – 60 = 27.5 amperes is added at a 25% demand factor.

Figure 5–8. Example of Farm Load Calculation

220.102

In the case of building No. 2, the nondiverse load is only 8.3 amperes; therefore, 51.7 amperes of “other” loads must be added at the 100% demand factor in order to meet the 60-ampere minimum.

Using the method given for computing total farm load, we see that the service load is

The total service load of 269 amperes requires the ungrounded service-entrance conductors to be at least 300-kcmil type THW copper conductors. The neutral load of the dwelling was assumed to be 100 amperes which brought the total farm neutral load to 207 amperes.

The farm dwelling load can be calculated by the standard method of calculation in all cases. The optional method of calculation for a dwelling could be used instead of the standard method unless the dwelling has electric heat and the farm has electric systems for drying grain.

QUIZ 5–2 (Closed-Book)

1. A reduced demand is allowed for commercial kitchen equipment when the number of units is: (a) Two or more (b) Three or more (c) Four or more 2. Using the optional method for a restaurant with electric cooking, the service demand load for a 586-kVA connected load is: (a) 468 kVA (b) 303 kVA (c) 410 kVA 3. The demand factor for diversified loads in a farm building for a load greater than 60 amperes is: (a) 75% (b) 25% (c) 50% 4. Select the occupancies in which a portion of the general lighting load may be reduced by applying demand factors: (a) Motels (b) Office buildings (c) Warehouses (d) Stores

(Open-Book)

1. Calculate the service load for a small restaurant that contains the following: (a) 1500 square feet of floor area (b) Eight duplex receptacles (c) Cooking equipment: (1) 20-kilowatt, 240-volt, three-phase range (2) 1½-horsepower, 240-volt, three-phase food chopper (3) 10-kilowatt, 240-volt, three-phase deep fat fryer (4) 4-kilowatt, 240-volt, single-phase food warmer (5) 5-kilowatt, 240-volt, single-phase dishwasher (6) 14-horsepower, 120-volt, single-phase vegetable peeler (7) 2.4-kilowatt, 240/120-volt toaster (8) 10-kilowatt, 240/120-volt waffle iron The service is a 240/120-volt, three-phase, four-wire service. Type THW copper conductors are used. Balance the loads as closely as possible. Use the standard method to select the service conductors and overcurrent protective devices. A comparison with the results of the optional method would show no substantial reduction in the size of the service for such a small restaurant. 2. Calculate the service load for a farm with the following loads: (a) 60-ampere dwelling load (b) 80-ampere load in building No. 1 (c) 40-ampere load in building No. 2 (d) 70-ampere load in building No. 3 3. Design the service and feeders for a 100-unit motel. The service is a 480-volt, three-phase circuit with a 480 to 208Y/120-volt transformer for the guest room service. Each guest room contains the following: (a) 240 square feet of floor area (b) 2-ampere, 208-volt air handler The motel contains a utility room with the following equipment: (a) 130-ampere, 208-volt, three-phase motor-compressor load; the locked-rotor current is 624 amperes (b) Ten 120-volt outside duplex receptacles Determine the motor branch-circuit rating and the size of the disconnecting means, the transformer kilovolt-amperes and feeder sizes, and the service rating. Type THW copper conductors are used for all circuits.

5–3 Special Occupancies and Equipment

The Code provides specific rules for designing feeders and branch circuits for certain special occupancies and equipment. These special rules apply to the design of wiring systems for hospitals, for mobile homes and mobile home parks, and for marinas and boatyards. These occupancies are discussed in some detail in this section. The wiring design rules for all other special occupancies and equipment are summarized in Table 5–1 at the end of this section for reference purposes.

5–3.1 Service Calculation for a Small Hospital

ARTICLE 517

The design of the electrical system for health care facilities is a complex task which is usually undertaken by designers who specialize in such systems. The calculation of the service load, however, is similar to that for any commercial occupancy.

Figure 5–9. Service Calculation for Small Hospital

An electrical system in a hospital consists of feeders and branch circuits supplying (a) nonessential loads, (b) equipment systems, and (c) the emergency system. The emergency system includes the life safety branch and the critical branch. Only the general lighting load, which is a part of the nonessential loads, may be reduced by applying the demand factors listed in the Code, but these demand factors do not apply to areas where the entire lighting capacity is likely to be used at one time, such as the operating room.

Simplified Design Example. The feeder load for a small hospital consists of the following continuous loads:

a. 100-kilovolt-ampere general lighting load in ward and administrative areas
b. 100-kilovolt-ampere load for life safety branch
c. 50-kilovolt-ampere load for critical branch
d. 25-horsepower, 480-volt, three-phase motor equipment system load

220.42

The loads are balanced on a 480-volt, three-phase service. It is required to determine the service ampacity as shown in Figure 5–9. Since each load is given as a continuous load value, an additional 25% need not be added. The 100-kilovolt-ampere lighting load represents a demand load of only 30,000 volt-amperes after the application of demand factors. The total load except for the equipment system motor load is 180 kilovolt-amperes, or 216.5 amperes per phase.

TABLE 430.250

The 25-horsepower motor has a full-load current of 34 amperes and its load is 1.25 × 34 amperes = 42.5 amperes. The required service ampacity is the sum of these loads, or 259 amperes.

5–3.2 Power Supply and Feeder Design for Mobile Homes, Manufactured Homes, and Mobile Home Parks

ARTICLE 550

A mobile home or a manufactured home is a factory-assembled structure that is capable of being transported to a site where it serves as a dwelling. The mobile home is designed for use as a permanent dwelling although its location may be moved on occasion. The manufactured home is typically not moved after it is placed on the home site. In most cases, the Code uses the term mobile home to include manufactured homes also.

550.32

In mobile or manufactured homes, the design of the branch circuits is the responsibility of the manufacturer. Only the design of the feeder or service circuit to a mobile home or to a mobile home park will be discussed here. One difference between a mobile home and a manufactured home is that the service equipment cannot be located in or on a mobile home but may be installed in or on a manufactured home if certain conditions are met.

550.10,

550.33

Design of Power Supply for Single Mobile Home. The power supply for a mobile home may be a single approved 40- or 50-ampere power supply cord if the load does not exceed the rating of the cord. If a larger load is to be served, a permanently installed circuit must be used.

550.18

ARTICLE 220

The Code provides a separate calculation method that must be used for computing the supply cord or distribution panel load for a mobile home. The lighting, small appliance, and laundry circuit loads are calculated in basically the same manner as before. The nameplate rating in amperes of all other appliances and motors and 25% of the load of the largest motor are added to determine the total load on the 120/240-volt service. The service rating is then based on the higher current calculated for either ungrounded conductor.⁴

⁴An optional method of calculation for mobile home lighting and appliance load is provided by the Code.

550.18(B)(5)

If a free-standing range is installed, the load current for the range is calculated by using a Code table of reduced values. In addition, the load of four or more appliances, excluding motors, heater loads, or free-standing ranges, may be reduced to 75% of the connected load.

The calculations for the power supply to a mobile home are shown in Figure 5–10 for a mobile home containing the following loads.

Figure 5–10. Example of Power Supply Calculations for Mobile Home

a. Outside dimensions of 60 feet by 12 feet
b. 500-watt, 120-volt dishwasher
c. 3-ampere, 120-volt garbage disposal
d. 10-kilowatt electric range
e. 1000-watt, 240-volt heater

550.18(A)

The lighting load at 3 volt-amperes/square foot, the two small appliance circuits, and the laundry circuit contribute a load of 6660 volt-amperes. This load is then reduced to a demand load of 3000 volt-amperes plus 35% of the remainder, or a total of 4281 volt-amperes. This load must be divided by 240 volts in order to find the load in amperes on each ungrounded conductor.

550.18(B)

The loads in amperes for motors and appliances are added to the load obtained for the lighting and small appliance load. The 120-volt loads should balance as closely as possible on the circuit. In this case, the only 120-volt loads are the dishwasher and garbage disposal with loads of 4.2 amperes and 3 amperes, respectively.

550.18(B)

The Code method also requires that 25% of the largest motor load be added to the total load but lists specific types of motor loads for which the requirement applies. The garbage disposal is not included in the list.

The hot-water heater has a load current of 4.2 amperes. Since there are only three appliances of this group, that is, the water heater, the garbage disposal, and the dishwasher, no reduction in the total load is allowed.

550.18(B)(5)

The load for the 10-kilowatt free-standing range is based on 80% of its rating and is 33.3 amperes.

550.10

The total load is 59.5 amperes on line A and 58.3 amperes on line B. Since these values are higher than the 50-ampere maximum for an approved power supply cord, a permanently installed power supply circuit is required.

550.31

TABLE 550.31

Mobile Home Park Service Equipment. The minimum rating for a service supplying a mobile home park is based on 16,000 volt-amperes (at 120/240 volts) per mobile home. A table of demand factors is provided that reduces this load according to the number of mobile home sites in the park.

As an example, if the park contains 21 mobile home sites, the load per site is

.25 × 16,000 VA = 4000 VA/site

The total service load is then

21 sites × 4000 VA/site = 84,000 VA

550.33(B)

TABLE 310.16

In any case, the minimum rating of each mobile home service is required to be at least 100 amperes. The conductors are selected from the Code table of ampacities.

5–3.3 Power Supply and Feeder Design for Recreational Vehicles and Recreational Vehicle Parks

ARTICLE 551

551.44

A recreational vehicle is a unit designed for temporary living and includes travel and camping trailers, truck campers, and motor homes. Since these vehicles are usually factory wired, the branch-circuit design is not covered in this Guide. The main power supply assembly that supplies power to the vehicle may be rated from 15 amperes to 50 amperes, depending on the number of branch circuits and the total load of the vehicle.

551.73

The rating of the 120/240-volt service for a recreational vehicle park is determined by the number of recreational vehicle sites in the park. The basic load is 3600 volt-amperes per site when equipped with both 20- and 30-ampere supply facilities and 2400 volt-amperes per site if there is only a 20-ampere supply. The load for a site with a 50-ampere supply is 9600 volt-amperes. This total load may be reduced when four or more sites are served.

For example, the total load for 50 sites with both 20-ampere and 30-ampere supplies is

50 × 3600 VA = 180,000 VA

TABLE 551.73

A demand factor of 41% may be applied, which results in a demand load of

.41 × 180,000 VA = 73,800 VA

This is the required service capacity for the 50-site park.

5–3.4 Shore Power Circuits for Marinas and Boatyards

ARTICLE 555

The wiring system for marinas and boatyards is designed by using the same Code rules as for other commercial occupancies except for the application of several special rules dealing primarily with the design of circuits supplying power to boats.

555.19

The smallest sized receptacle that may be used to provide shore power for boats is 30 amperes. Each single receptacle that supplies power to boats must be supplied by an individual branch circuit with a rating corresponding to the rating of the receptacle.

555.12

The feeder or service ampacity required to supply the receptacles depends on the number of receptacles and their rating, but demand factors may be applied that will reduce the load of five or more receptacles. For example, a feeder supplying ten 30-ampere shore power receptacles in a marina requires a minimum ampacity of

10 × 30 A × .8 = 240 A

Although this computed feeder ampacity might seem rather large, this is the minimum required by the Code.

5–3.5 Miscellaneous Wiring System Design Rules

Table 5–1 lists wiring system design rules for various types of equipment, occupancies, or systems that are not otherwise discussed in any detail in this Guide. The specific rules listed in the table pertain to ratings of circuits, conductors, and equipment and not to installation methods or other Code requirements. These rules should be read and understood fully in order to complete your study of wiring system design.

QUIZ 5–3 (Closed-Book)	1. The lighting load for mobile homes and park trailers is ________ volt-amperes/square foot.
	2. The mobile home service load before any demand factors are applied is ________ volt-amperes per mobile home site.
	3. For a mobile home or park trailer, the lighting and small appliance load over 3000 volt-amperes may be reduced by a demand factor of ________ percent.
	4. The feeder or service load for circuits supplying power to boats in a marina is based on:
	(a) The rating of the receptacles
	(b) The total load to be served
	(c) The branch-circuit ratings
	5. The branch-circuit conductors supplying data-processing equipment must have an ampacity based on the connected load of:
	(a) 100%
	(b) 125%
	(c) 150%
	6. The feeder circuit in a mobile home may be a power supply cord if the load does not exceed:
	(a) 40 amperes
	(b) 60 amperes
	(c) 50 amperes
(Open-Book)	1. What is the feeder load for a mobile home with the following loads?
	(a) 50-feet by 10-feet outside dimensions
	(b) Laundry area
	(c) 1000-watt, 240-volt heater
	(d) 5-kilowatt electric range
	(e) 1-kilowatt, 120-volt water heater
	(f) 500-watt, 120-volt dishwasher
	(g) 500-watt, 120-volt garbage disposal
	(h) 7-ampere, 240-volt air conditioner
	2. What service load is required for a mobile home park with 20 homes?
	3. A recreational vehicle has a 20-ampere air-conditioning circuit and two 15-ampere circuits. What rating is required for the power supply assembly?
	4. A recreational vehicle park consists of the following:
	(a) 30 sites with both 20-ampere and 30-ampere receptacles
	(b) 20 sites with 20-ampere power supply facilities
	What is the total service load for the park?
	5. What service capacity is required to supply five 30-ampere receptacles in a marina?